search: make references searchable

Allow searching for message that have the given message-id in
References: or Reply-To:

lib/message/mu-fields.hh

@@ -397,11 +397,12 @@ static constexpr std::array<Field, Field::id_size()>
 	    {
 		Field::Id::References,
 		Field::Type::StringList,
-		"references", {},
+		"references", "ref",
 		"References to related messages",
 		{},
 		'r',
 		Field::Flag::Value |
+				Field::Flag::BooleanTerm |
 		Field::Flag::IncludeInSexp
 	    },
 	    {

mu/mu-cmd-info.cc

@@ -106,8 +106,6 @@ topic_fields(const Options& opts)
 			return "boolean";
 		if (field.is_phrasable_term())
 			return "phrase";
-		if (field.is_value())
-			return "yes";
 		if (field.is_contact())
 			return "contact";
 		if (field.is_range())